Positive Redox Potential Research Articles

His-Ligation to the [4Fe-4S] Subcluster Tunes the Catalytic Bias of [FeFe] Hydrogenase.

[FeFe] hydrogenases interconvert H2 into protons and electrons reversibly and efficiently. The active site H-cluster is composed of two sites: a unique [2Fe] subcluster ([2Fe]H) covalently linked via cysteine to a canonical [4Fe-4S] cluster ([4Fe-4S]H). Both sites are redox active and electron transfer is proton-coupled, such that the potential of the H-cluster lies very close to the H2 thermodynamic potential, which confers the enzyme with the ability to operate quickly in both directions without energy losses. Here, one of the cysteines coordinating [4Fe-4S]H (Cys362) in the [FeFe] hydrogenase from the green algae Chlamydomonas reinhardtii ( CrHydA1) was exchanged with histidine and the resulting C362H variant was shown to contain a [4Fe-4S] cluster with a more positive redox potential than the wild-type. The change in the [4Fe-4S] cluster potential resulted in a shift of the catalytic bias, diminishing the H2 production activity but giving significantly higher H2 oxidation activity, albeit with a 200 mV overpotential requirement. These results highlight the importance of the [4Fe-4S] cluster as an electron injection site, modulating the redox potential and the catalytic properties of the H-cluster.

Multi-method assessment of the intrinsic biodegradation potential of an aquifer contaminated with chlorinated ethenes at an industrial area in Barcelona (Spain)

The bioremediation potential of an aquifer contaminated with tetrachloroethene (PCE) was assessed by combining hydrogeochemical data of the site, microcosm studies, metabolites concentrations, compound specific-stable carbon isotope analysis and the identification of selected reductive dechlorination biomarker genes. The characterization of the site through 10 monitoring wells evidenced that leaked PCE was transformed to TCE and cis-DCE via hydrogenolysis. Carbon isotopic mass balance of chlorinated ethenes pointed to two distinct sources of contamination and discarded relevant alternate degradation pathways in the aquifer. Application of specific-genus primers targeting Dehalococcoides mccartyi species and the vinyl chloride-to-ethene reductive dehalogenase vcrA indicated the presence of autochthonous bacteria capable of the complete dechlorination of PCE. The observed cis-DCE stall was consistent with the aquifer geochemistry (positive redox potentials; presence of dissolved oxygen, nitrate, and sulphate; absence of ferrous iron), which was thermodynamically favourable to dechlorinate highly chlorinated ethenes but required lower redox potentials to evolve beyond cis-DCE to the innocuous end product ethene. Accordingly, the addition of lactate or a mixture of ethanol plus methanol as electron donor sources in parallel field-derived anoxic microcosms accelerated dechlorination of PCE and passed cis-DCE up to ethene, unlike the controls (without amendments, representative of field natural attenuation). Lactate fermentation produced acetate at near-stoichiometric amounts. The array of techniques used in this study provided complementary lines of evidence to suggest that enhanced anaerobic bioremediation using lactate as electron donor source is a feasible strategy to successfully decontaminate this site.

Electrochemical properties of a series of Co(II) complexes, containing substituted phenanthrolines

Electrochemical studies of a series of substituted phenanthroline-Co(II) complexes all show generally similar behaviour, namely a chemically and electrochemically reversible CoIII/II redox couple, as well as a chemically and electrochemically reversible CoII/I redox couple, followed by a ligand-based reduction. Electron donating- or -withdrawing substituents on the phenanthroline ligands which are coordinated to the Co metal, directly influence the electron density on the Co metal, due to good communication between these substituents and the Co metal via the aromatic rings of the heterocyclic substituted phenanthroline-Co(II) complexes, leading to either more negative (for electron donating groups) or more positive (for electron withdrawing groups) redox potentials respectively. Linear relationships relating E°′(CoIII/II) oxidation and E°′(CoII/I) reduction to various experimental and empirical values, as well as to theoretically calculated energies, show that the electron density on Co is linearly influenced by the electronic properties of the ligands attached to the Co metal. All these established relationships can be used in the design of new substituted phenanthroline-Co(II) complexes with specific customized redox properties as required, for example, for the application of such Co(II) complexes as redox mediator for dye-sensitized solar cells.

Electrodeposited ZnO-Based, Organic-Dye Sensitized Solar Cells: Advantages and Limitations

The dye-sensitized solar cell (DSSC) is one of the most promising third generation photovoltaic technologies, which nowadays shows conversion efficiencies as high as 14 %. The attractive qualities of DSSCs include promising efficiencies, but also the low cost projected for their large-scale production for niche applications, in particular, for semi-transparent windows and indoor lighting conditions. This implies the optimization of all processing steps and components used, and a better understanding of the mechanisms of generation and loss of electrons involved in the solar cell functioning. Based on these observations, we have selected ZnO as the electron transporting material, not only for its much better electrical properties (when comparing with the commonly used TiO2), but also for the simple synthesis methods by which it can be obtained, such as electrodeposition (a low temperature an scalable technique), which offers to move toward the fabrication of more economically profitable devices. Hence, part of this work presents a detailed study on the effect of the bath composition, additives and deposition conditions used on the morphological and structural features of materials electrodeposited from ZnCl2 and Zn(NO3)2 solutions. The combination of this material with organic dyes with high absorption coefficient in the solar spectrum regime, promise an increase in cell current and voltage. For that reason, we have prepared solar cells with nanostructured, mesoporous ZnO films electrodeposited from an optimized aqueous ZnCl2 solution, an organic fluorenyl-thiophene dye (OD-8) as sensitizer, and an electrolyte solution with either the I-/I3 - or the [Co(2,2'-bipyridyl)3]2+/3+ redox couple. An improvement in efficiency may be expected, related to the more positive redox potential of the [Co(2,2'-bipyridyl)3]2+/3+ couple and faster dye regeneration kinetics, but this can only be achieved if the recombination rate at the semiconductor/electrolyte interface is sufficiently slow. Hence, we have evaluated the effect of CDCA (chenodeoxycholic acid) as coabsorbent in the dye solution, with the purpose to diminish the amount of uncovered area of the electrode surface, in order to minimize recombination. We use I-V curves in combination with intensity-modulated photovoltage spectroscopy, charge extraction measurements, and surface photovoltage spectroscopy to gain insight in the factors involved into the photochemical charge separation in ZnO-based organic-DSSCs.

Exogenously applied salicylic acid maintains redox homeostasis in salt-stressed Arabidopsis gr1 mutants expressing cytosolic roGFP1

Exogenous salicylic acid (SA) can be used for chemical hardening to alleviate oxidative stress in plants exposed to salinity. The treatment of 5-week-old Arabidopsis thaliana plants with increasing doses of SA alters the ascorbate (ASC) and glutathione (GSH) pools, and modulates their redox status and the activity of several antioxidant enzymes, such as ascorbate peroxidase (APX) and glutathione reductase (GR). To investigate the role of GR in the maintenance of cytoplasmic redox homeostasis after hardening by SA, wild type (WT) and gr1 mutant plants, expressing the cytoplasmic redox-sensitive green fluorescent protein (c-roGFP1), were pre-treated with 10−7 and 10−5 M SA for 2 weeks and subsequently exposed to 100 mM NaCl. The redox status of the salt-stressed WT plants became more oxidized, which was prevented by pretreatment with 10−5 M SA. The gr1 mutants showed more positive redox potential than WT plants, which could be reversed by treatment with 10−5 M SA. In mutants, the increased GSH levels may have compensated for the deleterious effect of GR deficiency and stabilized the redox potential in plants exposed to salinity. The ASC regeneration in WT plants shifted from the GSH-dependent dehydroascorbate reductase (DHAR) reaction to the NAD(P)H-dependent monodehydroascorbate reductase (MDHAR) activity during chemical hardening, which contributed to the preservation of the GSH pool in plants under salt stress. Our results suggest that the maintenance of GSH levels and redox homeostasis by SA-mediated hardening play a major role in priming and defending against salt stress.

Charge Transfer Characteristics of Diaza-Anthraquinones in Different Solvents and Their Application As Organic Active Material in Redox Flow Batteries

Redox flow batteries (RFBs), with their independent scalability of power and capacity and their versatility concerning applicable active materials and solvents, pose a promising technology to meet the challenges of cost efficient large scale stationary energy storage. This holds particularly for RFBs based on the redox active material class of quinones that can be derived from naturally abundant resources at low cost. In this work, we electrochemically investigate the modified quinone 2,3-diaza-anthracenedione, and two of its derivatives, as promising active material for aqueous redox flow batteries. Interestingly, these anthraquinone derivatives exhibit varying charge transfer properties depending on their functionalization and the applied solvent and pH value. In general, a positive redox potential shift of about 300 mV is achieved by the incorporation of a diaza moiety into the anthraquinone base structure. Moreover, our experiments at low pH prove that the addition of a methoxy group to the base structure of the 2,3-diaza-anthracenedione strongly increases the electrochemical stability in aqueous acidic media. A functionalization with two hydroxyl groups in close vicinity to the carbonyl group evokes a negative redox potential shift of 54 mV in acidic and 264 mV in alkaline solution. For this study the influence of protonation and hydrogen bonding on the charge transfer characteristics of diaza-anthraquinones was investigated specifically by comparison of measurements taken in non-aqueous solvents and in aqueous solutions at high and low pH. Whereas at low pH values solely one voltammetric anodic and cathodic wave can be observed for the occurring two-electron transfer, peak splitting takes place at high pH values for two of the derivatives, which is highly unusual for quinonoid compounds. Further, measurements in non-aqueous solvents reveal a greater potential difference between the two subsequent electron transfers compared to the pristine anthraquinone. This indicates a discrepancy in charge transfer kinetics evoked by the incorporation of a diaza-moiety into the anthraquinone base structure. Our findings allow for a deeper understanding of the redox characteristics of diaza-functionalized quinonoid compounds that could lead up to efficient active materials for future redox flow batteries with organic active material. Figure 1

Quest for Organic Active Materials for Redox Flow Batteries: 2,3-Diaza-anthraquinones and Their Electrochemical Properties

To be competitive with other electrically rechargeable large scale energy storage, the range of active materials for redox flow batteries is currently expanded by organic compounds—this holds especially for the redox active material class of quinones that can be derived from naturally abundant resources at low cost. Here we propose the modified quinone 2,3-diaza-anthracenedione, and two of its derivatives, as a promising active material for aqueous redox flow batteries. We systematically study the electrochemical performance (redox potentials, rate constants, diffusion coefficients) for these three compounds at different pH values experimentally and complement the results with density functional calculations: A positive redox potential shift of about 300 mV is achieved by the incorporation of a diaza moiety into the anthraquinone base structure. Our experiments at low pH show that the addition of a methoxy group to the base structure of the 2,3-diaza-anthracenedione strongly increases the electrochemical ...

Growth of Leptospirillum ferriphilum in sulfur medium in co-culture with Acidithiobacillus caldus

Leptospirillum ferriphilum and Acidithiobacillus caldus are both thermotolerant acidophilic bacteria that frequently co-exist in natural and man-made environments, such as biomining sites. Both are aerobic chemolithotrophs; L. ferriphilum is known only to use ferrous iron as electron donor, while A. caldus can use zero-valent and reduced sulfur, and also hydrogen, as electron donors. It has recently been demonstrated that A. caldus reduces ferric iron to ferrous when grown aerobically on sulfur. Experiments were carried out which demonstrated that this allowed L. ferriphilum to be sustained for protracted periods in media containing very little soluble iron, implying that dynamic cycling of iron occurred in aerobic mixed cultures of these two bacteria. In contrast, numbers of viable L. ferriphilum rapidly declined in mixed cultures that did not contain sulfur. Data also indicated that growth of A. caldus was partially inhibited in the presence of L. ferriphilum. This was shown to be due to greater sensitivity of the sulfur-oxidizer to ferric than to ferrous iron, and to highly positive redox potentials, which are characteristic of cultures containing Leptospirillum spp. The implications of these results in the microbial ecology of extremely acidic environments and in commercial bioprocessing applications are discussed.

The interaction of pyridoxal isonicotinoyl hydrazone (PIH) and salicylaldehyde isonicotinoyl hydrazone (SIH) with iron

The interaction of pyridoxal isonicotinoyl hydrazone (PIH) and salicylaldehyde isonicotinoyl hydrazone (SIH), two important biologically active chelators, with iron has been investigated by spectrophotometric methods. High iron(III) affinity constants were determined for PIH, logβ2=37.0 and SIH, logβ2=37.6. The associated redox potentials of the iron complexes were determined using cyclic voltammetry at pH7.4 as +130mV (vs normal hydrogen electrode, NHE) for PIH and +136mV(vs NHE) for SIH. These redox potentials are much higher than those corresponding to iron chelators in clinical use, namely deferiprone, −620mV; desferasirox, −600mV and desferrioxamine, −468mV. Although the positive redox potentials of SIH and PIH are similar to that of EDTA, namely +120mV, the iron complexes of these two hydrazone chelators, unlike the iron complex of EDTA, do not redox cycle in the presence of vitamin C. These properties render PIH and SIH as excellent scavengers of iron, under biological conditions. Both SIH and PIH scavenge mononuclear iron(II) and iron(III) rapidly. These fast kinetic properties of the hydrazone-based chelators provide a ready explanation for the adoption of SIH in fluorescence-based methods for the quantification of cytosolic iron(II).

The effect of Lobelia dortmanna L. on the structure and bacterial activity of the rhizosphere

We demonstrate the impact of Lobelia dortmanna on the abundance and morphological structure and function of the bacterial biocoenosis occurring in the rhizosphere sediments. The metabolic activity of microorganisms, the intensity of respiration, the efficiency of respiratory chain and membrane integrity was evaluated. The significant influence of L. dortmanna on microbiological processes in the sediments was confirmed. Clearly the high amount, biomass and metabolic and physiological activity of bacteria in the rhizosphere sediments confirmed the stimulating effect of isoetids on sediment bacteria. The rhizosphere is inhabited by microorganisms which are very active in physiological and metabolic terms. This phenomenon is called the rhizosphere effect. The amount of carbon dioxide released in particular sediment zones was predominantly determined by the number of metabolically active cells and not by the respiration rate per single cell. The high respiration rate of bacteria in the rhizosphere was predominantly determined by high DOC concentration, as evidenced by the correlations between the parameters. The development of positive redox potential in the rhizosphere sediments by L. dortmanna was of considerable importance for the intensity of respiratory processes, which confirms the relationship between redox potential and CO2 concentration in the sediments. The specific system of functional relationships occurring between the roots of L. dortmanna and microorganisms present in bottom sediments is one of the important mechanisms enabling the well-functioning of this macrophyte in the littoral zone of soft-water oligotrophic lakes.

Low levels of lead and glutathione markers of redox status in human blood.

Exposure to lead (Pb) is implicated in a plethora of health threats in both adults and children. Increased exposure levels are associated with oxidative stress in the blood of workers exposed at occupational levels. However, it is not known whether lower Pb exposure levels are related to a shift toward a more oxidized state. To assess the association between blood lead level (BLL) and glutathione (GSH) redox biomarkers in a population of healthy adults, BLL and four GSH markers (GSH, GSSG, GSH/GSSG ratio and redox potential E h ) were measured in the blood of a cross-sectional cohort of 282 avid seafood-eating healthy adults living on Long Island (NY). Additionally, blood levels of two other metals known to affect GSH redox status, selenium (Se) and mercury (Hg), and omega-3 index were tested for effect modification. Regression models were further adjusted for demographic and smoking status. Increasing exposure to Pb, measured in blood, was not associated with GSSG, but was associated with lower levels of GSH/GSSG ratio and more positive GSH redox potential E h , driven by its association with GSH. No effect modification was observed in analyses stratified by Hg, Se, omega-3 index, sex, age, or smoking. Blood Pb is associated with lower levels of GSH and the GSH/GSSG ratio in this cross-sectional study of healthy adults.

Electrochemical Potential Gradient As a Quantitative Test Platform for Cellular Oxidative Stress

Oxidative stress (OS), involved in many biological phenomena, such as aging, carcinogenesis and inflammation, is defined as an imbalance between reactive oxygen species (ROS) and organism’s antioxidant defenses. The consequences of this imbalance lead to the disruption of cellular redox homeostasis and oxidative damage to DNA, proteins and lipids.(1) One metric of the relative strengths of oxidizers implicated in causing pathogenic oxidative damage via ROS production is the effective redox potential experienced by the cells. Therefore, establishing the relationship between the strength of the oxidative action, characterized by redox potential, and the biomolecular consequences is critical to elucidating the thermodynamic framework of oxidative damage. Experiments with OS in the lab environment require subjecting the study object to a given level of oxidative bias relative to biosystem’s natural equilibrium, preferably in a well-defined, reproducible and quantitative fashion. Popular in vitro OS inducers include exogenous chemical agents (hydrogen peroxide, paraquat, menadione, and t-butyl hydroperoxide), ionizing radiation or heat shock. In addition to disrupting redox homeostasis, each of these agents will also have a specific chemical interaction with cell components, which have to be accounted for during interpretation.(2) Here, we describe a novel way to create an OS environment for surface attached Chinese Hamster Ovary (CHO) cells by exposing them to an electrochemical potential gradient. With this technique we can apply a stable linear potential profile over a cell monolayer, thus providing an efficient method to screen live cell response to an exogenous redox background. It is based on a bipolar electrode configuration (3), adapted for a rectangular cell growth flask. A constant current is produced in cell media, thus generating an electric field and potential drop over the mammalian cell monolayer. Surface attached cells experience varying electrochemical potential, depending on their location along the current axis. Subsequently, cell viability gradients were produced within a range of treatment levels. The live-dead assay fluorescence image of the flask bottom slide shows the CHO cell condition following incubation under the redox potential gradient for a fixed time interval (see Figure). Images contain a rather uniform live, dead and a narrow changeover region ( magnified view in the lower panel of Figure ), consistent with a commencement of cell death at increasingly positive solution redox potential. The cell viability ratio was calculated from integrated pixel density using a custom ImageJ software macro. Therefore, the sigmoidal cell viability curve shape, produced when cells are exposed to the electrochemical potential gradient , is consistent with a two state conversion model and could be numerically characterized by a half maximum effective potential. This concept is widely used to describe dose-response curves in toxicology. Such numerical values would be useful to compare cell viability when subjected to oxidative stress in diverse experimental circumstances. That way, the higher effective potential value would indicate the increase in cell viability or the efficacy of the antioxidant. We have compared CHO cell response when exposed to three hydrogen peroxide concentrations (20 mM, 50 mM, and 100 mM) with the exposure to electrochemically induced potential gradient. Redox potential values of these H2O2 preparations were measured in separate experiments 1h after hydrogen peroxide was diluted into the cell growth media. Remarkably, cell viability vs solution redox potential dependencies are very close suggesting that exogenous hydrogen peroxide causes CHO cell OS largely by a positive shift in the solution redox potential, at least in this concentration range. Our test platform allows for direct imaging and analysis of cell viability, exposed to a range of electrochemical potentials. The slide vessel system can readily be adapted for other cellular assays such as ROS detection, apoptosis, etc. The system is particularly useful for providing a linear range of oxidative and reductive potentials in a single experiment. By replacing the vessel contents before during and after treatment a wide range of solution conditions can be tested and cell images can be acquired during oxidative treatment. This is particularly useful for in vitro testing of the effectiveness of additives such as anti-oxidants as it enables monitoring a real time cellular response. In addition, various cell types including cancer cell lines can be compared in their resistance to oxidative potentials and effectiveness of various drugs. References (1) Halliwell, B. Trends Pharmacol Sci 2013, 34, 301. (2) Buettner, G. R.; Wagner, B. A.; Rodgers, V. G. J. Cell Biochem Biophys 2013, 67, 477. (3) Fleischmann, M.; Ghoroghchian, J.; Rolison, D.; Pons, S. J Phys Chem-Us 1986, 90, 6392. Figure 1

Synthesis of iron and copper cluster-grafted zinc oxide nanorod with enhanced visible-light-induced photocatalytic activity

Design of visible-light-responsive photocatalyst employing simple and cost-effective method is of great importance from commercial point of view. Herein, we report the synthesis of visible-light-sensitive ubiquitous nanoclusters of Fe3+/Cu2+-grafted ZnO nanorod using impregnation technique, which showed excellent photocatalytic activity towards the decomposition of Rhodamine B (RhB), 4-nitrophenol (4-NP) and paracetamol in aqueous suspension under atmospheric oxygen. Fe-grafted ZnO nanorod exhibited pronounced effect for the degradation of the above-mentioned pollutants compared to pure ZnO and Cu-grafted ZnO nanorod. The better activity could be due to the more positive redox potential of surface grafted Fe3+ species resulting in the generation of more hydroxyl radicals thereby, leading to higher photodegradation rate.

Design of Metal-to-Metal Charge-Transfer Chromophores for Visible-Light Activation of Oxygen-Evolving Mn Oxide Catalysts in a Polymer Film

To construct photoresponsive unidirectional charge transfer units for the activation of oxygen-evolving manganese oxide (MnOx) catalyst, metal-oxide nanoclusters consisting of cerium (CeIII) or cobalt (CoII) ions and Keggin-type polyoxotungstate (PW12O403–) were synthesized in a polymer matrix as visible-light-absorbing chromophores. The utilization of the polymer matrix enabled the molecularly dispersed PW12O403– states and was advantageous to achieve product-separable energy conversion systems. The reaction of PW12O403– with Ce(NO3)3 or CoCl2 in the polymer matrix generated the new broad absorption tails extending from the UV to visible region, assignable to metal-to-metal charge transfer (MMCT) transitions of oxo-bridged binuclear WVI–O–CeIII and WVI–O–CoII units. Cyclic voltammetry analysis of the oxo-bridged binuclear units in the polymer membrane revealed that the CoIII/CoII couple had a 300 mV more positive redox potential than that of CeIV/CeIII and was capable of extracting electrons from MnOx ca...

Structure, biochemical and kinetic properties of recombinant Pst2p from Saccharomyces cerevisiae, a FMN-dependent NAD(P)H:quinone oxidoreductase

The genome of the yeast Saccharomyces cerevisiae encodes four flavodoxin-like proteins, namely Lot6p, Pst2p, Rfs1p and Ycp4p. Thus far only Lot6p was characterized in detail demonstrating that the enzyme possesses NAD(P)H:quinone oxidoreductase activity. In the present study, we heterologously expressed PST2 in Escherichia coli and purified the produced protein to conduct a detailed biochemical and structural characterization. Determination of the three-dimensional structure by X-ray crystallography revealed that Pst2p adopts the flavodoxin-like fold and forms tetramers independent of cofactor binding. The lack of electron density for FMN indicated weak binding, which was confirmed by further biochemical analysis yielding a dissociation constant of 20±1μM. The redox potential of FMN bound to Pst2p was determined to −89±3mV and is thus 119mV more positive than that of free FMN indicating that reduced FMN binds ca. five orders of magnitude tighter to Pst2p than oxidized FMN. Due to this rather positive redox potential Pst2p is unable to reduce free FMN or azo dyes as reported for other members of the flavodoxin-like protein family. On the other hand, Pst2p efficiently catalyzes the NAD(P)H dependent two-electron reduction of natural and artificial quinones. The kinetic mechanism follows a ping-pong bi-bi reaction scheme. In vivo experiments with a PST2 knock out and overexpressing strain demonstrated that Pst2p enables yeast cells to cope with quinone-induced damage suggesting a role of the enzyme in managing oxidative stress.

Switchable Redox Chemistry of the Hexameric Tyrosine-Coordinated Heme Protein.

Hexameric tyrosine-coordinated heme protein HTHP from Silicibacter pomeroyi has been shown to exhibit peroxidase- and catalase-like activity. In the present study, detailed spectroscopic and electrochemical investigations were performed to analyze the redox properties and active site structure of HTHP. Potentiometric titration of HTHP in solution revealed a single redox transition at -0.54 V (vs Ag/AgCl), indicating six structurally identical tyrosine coordinates hemes. Cyclic voltammetry (CV) of immobilized HTHP afforded a distinctly more positive redox potential (-0.17 V) but failed to detect a transition at -0.54 V. Conversely, surface enhanced RR (SERR) spectroscopy provided evidence for both high- and low-potential transitions and for a partial loss of heme in the reduced state. The high-potential CV-active redox transition is attributed to the hemes of the barrel-shaped HTHP in a wheel-like orientation on the surface. Supported by coarse-grained simulations and SERR spectroscopy, the majority of HTHP is concluded to adopt a reverse-disc orientation, accounting for the low-potential transition. In view of the striking similarity of HTHP to the heme carriers HasA or HmbR regarding redox potential, Fe-Tyr ligation, and heme release, we propose heme transport as an alternative or additional function.

INVESTIGATION OF CHANGES OF PHYSICAL AND CHEMICAL PROPERTIES OF TAP WATER UNDER INFLUENCE OF POWERFUL UNDERWATER SPARK DISCHARGES

Purpose. The purpose of this investigation is to study the changes in the redox potential and pH-value of the tap water as a function of underwater spark discharges, storage device capacitance and the charging voltage. Methodology . To define the electric parameters of discharge circuit we used the Rogowski loop and the compensated capacitance-ohm potential divider. To determine water properties before and after the treatment we used the following devices: the water analyzer Anion -7051 with the limit of absolute error of the EMF measurement ± 2 mV, the BANTE 902 device with the absolute error of ± 0.002 рН. Results. We managed to establish the time of the origination of changes in the properties of treated water. A change in the positive redox potential to a negative one occurred already after the third pulse at a total energy input of ≥ 1 kJ. The pH value increased in the range of 0.2 – 0.45 рН units. We obtained the relationship of a change in the redox potential as a function of total energy of the pulse train that actually exhibits the linear relation to the mass of erosion products. We established that the electrodes made of stainless steel and the electrodes made of graphite provide similar changes in water properties. An increase in pH is indicative of the progress of reactions that result in the formation of ОН‾. Originality. The obtained experimental data prove a rapid and reliable change in the redox potential from positive to negative changes in the redox potential exponent can persist during ten days and even longer. The erosion products of electrodes can be removed from the treated water using the method of magnetic separation. Practical value. The «HYDRA» plant can be used as the electrochemical ideal mixing reactor of a discrete action with the removal of erosion products of the electrode using the method of magnetic separation.

Iron(ii) complexes of 2-mercaptopyridine as rubredoxin site analogues.

The synthesis, characterization and reactivity studies of iron(ii) complexes [FeII(PySH)4](OTf)2, 1-(OTf)2, [FeII(PySH)4](ClO)4, 1-(ClO4)2, and [FeII(PyS)2]n, (2), of a 2-mercaptopyridine (PySH) ligand are discussed. The X-ray crystal structures of both 1-(OTf)2 and 1-(ClO4)2 reveal a distorted tetrahedral geometry at the iron(ii) center with identical constituents. All the pyridine nitrogen atoms are protonated and thiolate ions are coordinated to the iron(ii) center. The structure and function of complex 1-(OTf)2 or 1-(ClO4)2 resembles the active site of rubredoxin. Complex 2 has octahedral geometry at the iron(ii) center forming a 1-D coordination polymer. Complex 1-(OTf)2 exhibits a high positive redox potential (E1/2 = 0.23 V vs. Ag/AgCl) which reduces to -0.12 V in the presence of triethylamine under an inert atmosphere. This change of the redox potential is highly reversible in the presence of a weak acid such as p-toluenesulfonic acid, pTsOH. DFT studies show that the complex cation [FeII(PySH)4]2+ upon treatment with a base converts to its anionic congener, [FeII(PyS)4]2-, via the deprotonation of the pyridinium moiety. The iron(ii) complexes readily react with molecular oxygen to yield the corresponding iron(iii) complex, which rapidly decays to form pyridine disulphide (Py2S2) and an iron(ii) complex.

Corrosion of copper in oxygen-deficient groundwater with and without deep bedrock micro-organisms: Characterisation of microbial communities and surface processes

Copper specimens were exposed to oxygen-deficient artificial groundwater in the presence and absence of micro-organisms enriched from the deep bedrock of the planned nuclear waste repository site at Olkiluoto island on the western coast of Finland. During the exposure periods of 4 and 10 months, the copper specimens were subjected to electrochemical measurements. The biofilm developed on the specimens and the water used in the exposures were subjected to microbiological analyses. Changes in the water chemistry were also determined and surfaces of the copper specimens were characterized with respect to the morphology and composition of the formed corrosion products. The results showed that under biotic conditions, redox of the water and open circuit potential (OCP) of the copper specimens were generally negative and resulted in the build-up of a copper sulphide, Cu2S, layer due to the activity of sulphate-reducing bacteria (SRB) that were included in the system. In the 4-month test, the electrochemical behaviour of the specimens changed during the exposure and alphaproteobactria Rhizobiales were the dominant bacterial group in the biofilm where the highest corrosion rate was observed. In the 10-month test, however, deltaproteobacteria SRB flourished and the initial electrochemical behaviour and the low corrosion rate of the copper were retained until the end of the test period. Under abiotic conditions, the positive water redox potential and specimen OCP correlated with the formation of copper oxide, Cu2O. Furthermore, in the absence of SRB, Cu2O provided slightly inferior protection against corrosion compared to that by Cu2S in the presence of SRB. The obtained results show that the presence of microorganisms may enhance the passivity of copper. In addition, the identification of key microbial species, such as SRB thriving on copper for long time periods, is important for successful prediction of the behaviour of copper.

Control of oxidation-reduction potential during Cheddar cheese ripening and its effect on the production of volatile flavour compounds.

In cheese, a negative oxidation-reduction (redox) potential is required for the stability of aroma, especially that associated with volatile sulphur compounds. To control the redox potential during ripening, redox agents were added to the salted curd of Cheddar cheese before pressing. The control cheese contained only salt, while different oxidising or reducing agents were added with the NaCl to the experimental cheeses. KIO3 (at 0·05, 0·1 and 1%, w/w) was used as the oxidising agent while cysteine (at 2%, w/w) and Na2S2O4 (at 0·05 and 0·1%, w/w) were used as reducing agents. During ripening the redox potential of the cheeses made with the reducing agents did not differ significantly from the control cheese (E h ≈ -120 mV) while the cheeses made with 0·1 and 0·05% KIO3 had a significantly higher and positive redox potential in the first month of ripening. Cheese made with 1% KIO3 had positive values of redox potential throughout ripening but no starter lactic acid bacteria survived in this cheese; however, numbers of starter organisms in all other cheeses were similar. Principal component analysis (PCA) of the volatile compounds clearly separated the cheeses made with the reducing agents from cheeses made with the oxidising agents at 2 month of ripening. Cheeses with reducing agents were characterized by the presence of sulphur compounds whereas cheeses made with KIO3 were characterized mainly by aldehydes. At 6 month of ripening, separation by PCA was less evident. These findings support the hypothesis that redox potential could be controlled during ripening and that this parameter has an influence on the development of cheese flavour.